At the present time there are two main methods of splicing optical fibers- fusion splicing and mechanical splicing.
Fusion splicing is performed by aligning the cores, or light propagating regions, of the two fibers so that minimal light loss occurs at the junction. Next, a flame or electrical arc is used to soften or melt the optical fibers to the point where they are welded or fused together. This technique is usually very operator sensitive, cannot be quickly performed, requires expensive and delicate equipment, and requires electrical power either via voltage or batteries. Furthermore, because the technique involves the use of a spark or flame, it poses a safety problem if used in hazardous environments such as manholes or mines.
Mechanical splicing can be subdivided into several different techniques: chip or array, crimp, epoxy and polish, four or three rod, and elastomeric.
Chip splicing involves the placement of a single fiber, or multiple (array) fibers, onto a substrate. This substrate has precision machined or etched v-grooves running longitudinally along its top surface. Each groove is of such dimension as to permit the fiber to rest within it. In preparation for splicing, a portion of each fiber's protective coating is removed, and the fiber ends are cleaved or polished to achieve a perpendicular fiber end face. One of the fibers to be spliced is inserted into its v-groove so that it extends halfway along the length of the substrate. The fiber that is to be spliced to the fiber in the groove is then inserted into the same groove until it butts against the first fiber. Next, the fibers are secured onto the chip using an adhesive. Once the adhesive has cured, the splice is complete.
Disadvantages of this technique are that the substrates are difficult to accurately manufacture, and the v-grroves do not permit the fiber cores to be accurately aligned via "optimization" or manual tuning.
The crimp type splice involves the same fiber end preparation as the chip splice. Once the fiber ends are prepared, an optical coupling compound is applied to each fiber end face, and the two fibers to be joined are centered, end-to-end, on a metal substrate that has several longitudinal ridges. After the fibers are positioned, the metal substrate is formed around the fibers using a special tool. This tool causes the metal substrate to hold the fibers securely in place and in proper alignment. The splice is then complete.
The crimp technique creates several conditions that can be detrimental to the fiber in terms of life and performance. These include: metal applying pressure to a glass optical fiber can introduce microcracks to the fiber, the bare fibers are not protected or sealed from the environment permitting moisture to attack them, and the fiber alignment cannot be optimized.
The epoxy and polish technique is very similar to fiber optic connector assembly. The coating on the fiber is removed at the ends. Each fiber is then inserted into a ferrule which contains a precision drilled hole at one end. The fiber is epoxied into the ferrule so that some bare fiber extends out of the precision drilled hole. Once the epoxy is cured, the fiber ends are polished perpendicular to the ferrule axis using abrasive techniques. The ferrules of the two fibers to be spliced are then inserted into a receptacle with a precision longitudinal bore filled with optical coupling compound. The fiber cores are then in alignment or the ferrules can be rotated until optimal alignment is achieved. Once aligned, the ferrules are held within the receptacle via mechanical clamps or nuts or adhesives.
The epoxy and polish splice technique has the disadvantage that it is expensive due to the precision of its three components, and the fiber end preparation is difficult to perform in the field and time consuming.
The three or four rod techniques of splicing are the same as the chip splicing except that the fibers are inserted into the interstitial space formed by three or four rigid rods that are secured together. The fibers are held into the rod assembly using an adhesive.
This technique makes optimization difficult, and in most cases impossible. Further, the joint may or may not be adequately sealed from the environment-leading to reliability problems.
The mechanical elastomeric splice is a technique shown and described in U.S. Pat. No. 4,257,674, and the teachings thereof are incorporated herein by reference, and involves the same coating removal and fiber cleaving technique as required for chip splicing. The fibers are bonded into the glass fiberguides of the splice with an ultraviolet curing adhesive after they are inserted into the splice and optimized. Prior to field installation, two resilient elastomer halfhexes, one of which has a precision, longitudinal, index matching gel filled, v-groove on its widest face, are inserted into the middle of a glass sleeve. Next, two precision glass fiberquides are inserted into the glass sleeve, one in each end, and bonded thereto. These fiberquides are available in three sizes, depending on the coating diameters of the fibers to be spliced. The "splice assembly" is then ready for field use.
The disadvantage of this technique is that the field installer must know the coating diameter of the fiber to be spliced in order that he can select the splice with the proper fiberquides. This is often very difficult to quickly determine, especially in emergency situations where access to cable/fiber information is very limited. Also, the adhesive used to bond the fibers into the fiberquides cannot wick fully into the fiberquides because of air trapped within the tube. This makes it possible to have an inadequate bond to the bare fiber, which can cause poor environmental performance. Similarly, the ultraviolet curing adhesive requires an ultraviolet light source, which may not be readily available in an emergency. Also, because the tube and fiberquides are glass, they can be easily broken or chipped if mishandled, thus making them unusable. This can be very expensive.